1. Field of the Invention
The present invention relates generally to battery chargers, and more particularly to an application specific integrated circuit (ASIC) that is configured to charge nickel cadmium (NiCad) battery cells.
2. Description of the Related Art
Many electronic devices rely on batteries for power. Since batteries are relatively expensive and need frequent replacement, rechargeable batteries, such as NiCad batteries, are used instead. When rechargeable batteries are used in devices that rely solely on batteries for power, such as small transistor radios, the rechargeable batteries must be periodically removed from the device and charged in a separate battery charger. However, many electronic devices include AC adapters and an internal battery charger. When the device is being externally powered by the AC adapter, the battery charger replenishes the rechargeable battery cells.
In many devices, the battery chargers measure the temperature of the rechargeable batteries and relate the temperature of the battery to the charge on the battery. Unfortunately, these types of charging devices are riddled with problems. First, temperature sensors must be implanted in each battery. This increases the manufacturing expense of each battery. Second, changes in the ambient temperature upset the correlation between the temperature of the battery and the charge on the battery. The battery charger could overcharge the battery in low ambient temperature conditions, or undercharge the battery in high ambient temperature conditions.
For instance, one type of temperature sensing battery charger delivers high charging current to batteries while the temperature of the batteries is below 40.degree. C. Once the temperature of the batteries exceeds 40.degree. C., the batteries are deemed fully charged. Therefore, the high current is replaced by a lower current to maintain the charge on the batteries while the device is in use. However, if the batteries are in an environment having a low ambient temperature, the high current could overcharge the batteries, and, thus, damage the batteries.
Even if battery damage does not occur, overcharged batteries exhibit a phenomenon known as "voltage depression". Voltage depression occurs when the batteries exceed 100% charge and the voltage across the batteries begins to fall. Since decreasing battery voltage is not a desirable function of a battery charger, battery chargers of this type exhibit clear disadvantages.
Moreover, batteries that remain fully charged, yet are seldom discharged exhibit "battery memory." Battery memory substantially decreases the operating life of a battery, because the battery discharges quickly. To avoid this problem, electronic devices containing rechargeable batteries must be periodically used without external power so that the batteries may discharge.